DNA-dependent RNA polymerase has been solubilized from rat liver mitochondria. The enzyme has been partially purified. The recovery of the enzyme (relative to the activity in swollen mitochondria) is approximately 75%. The enzyme will be purified extensively and its subunit composition will be determined. The properties of the purified enzyme will be compared with those obtained from bacteria and liver nuclei. Various DNA preparations and synthetic polydeoxynucleotides will be used as templates to determine the template specificity. The effects of specific inhibitors of RNA synthesis on the mitochondrial RNA synthesizing system will be examined. Finally, the regulation of RNA polymerase activity by hormones and diets will be investigated.